An Opportunity is a way for Brands to guage interest in a campaign they are considering running. You'll receive a brief description and category as well as the Opportunity to submit a rate that you would be interested in having a related Offer for.
For example you could receive an Opportunity that would say something like.... "A new horror movie is about to be released by a major studio, what rate would you be interested in posting about this movie for?"
You would submit your rate and now when the Brand creates the campaign they can know who they should contact first with an Offer. An Opportunity lets them generate interest and lets you get a chance to beat everyone else to the front of the line.